1. Field of the Invention
This invention relates generally to storage device servo control systems and, more particularly, to disk drive position control systems that determine the location of a head relative to disk tracks.
2. Description of the Related Art
In a conventional computer data storage system having a rotating storage medium, such as a magnetic or magneto-optical disk, data is stored in concentric or spiral tracks across the surface of the disk. Consumers are demanding disk storage systems with increasing capacity, generally at a rate of 60% greater capacity each year. The most expedient way of providing greater disk capacities has been to increase track density in the disk radial direction, meaning data is stored in increasingly narrow tracks. Greater data density in the radial direction is easier to achieve than increasing data density in the circumferential direction, longitudinally within the tracks. Thus, greater data density is achieved by decreasilg the track pitch to provide more tracks from a same-sized disk. With increasing disk capacity, it is becoming more and more difficult to economically maintain the manufacturing tolerances of read head width and write head width to keep pace with the increasingly smaller track pitch.
The data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprisinig reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent information. The binary digits must be read from and recorded onto the disk surface. A transducer such as a combined read/write head is provided to produce and detect the variations in magnetic orientation of the magnetic material as the disk rotates relative to the head. Conventionally, the read/write head is mounted on a disk arm that is moved across the disk by a servo.
A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head from data track to data track and, once over a selected track, to maintain the head in a track-following path over the centerline of the selected track. Maintaining the head centered over a track facilitates accurate reading and recording of data in the track. Positioning read/write heads is one of the most critical aspects of recording and retrieving data in disk storage systems. With the very high track density of current disk drives, even the smallest head positioning elTor can potentially cause a loss of data. Accordingly, a great deal of effort is devoted to the design of servo control systems.
A servo control system generally centers a read/write head over a track by reading servo information recorded onto the disk surface. The servo information comprises a position-encoded servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in data fields reserved for disk servo information. The servo pattern typically comprises bursts of flux transitions formed as parallel stripes in the servo tracks, oriented in the radial direction. When the read/write head passes over the servo pattern flux transitions, the head generates an analog readable signal whose repeating cyclic variations can be demodulated and decoded to indicate the position of the head relative to the track from which the pattern was read. The demodulated servo signal is referred to as a position error sensing (PES) signal.
In a sector servo method for providing servo track information to a disk servo control system, each track of the disk surface includes a sector of servo information interspersed between binary data fields. That is, the tracks on a sector servo disk surface are divided into sectors having a short servo track information area followed by a customer data area. The servo information area typically contains a sector marker track identification data and then one or more servo pattern bursts. The sector marker indicates the beginning of a servo sector to the data detection electronics so the data detection electronics will realize that servo information immediately follows in the track. The servo read head can be the same read head used for reading customer data or can be a separate, dedicated servo head. The PLS signal is used to generate a corrective input signal that is applied to the read/write head positioning servo.
The servo pattern is generally recorded such that a quadrature readback signal is generated. More particularly the servo pattern comprises bursts grouped into A, B, C and D bursts that are offset from each other and repeated at regular intervals in the disk tracks. The servo pattern is demodulated so as to generate two cyclic readback signal components a primary signal component called "P" and a quadrature signal component called "Q", each 90.degree. out of phase with the other. With respect to the readback signal generated when the read head is over the A, B, C or D servo bursts, the primary P signal component is formed from the relationship P=B-A and the quadrature Q signal component is formed from the relationship Q=D-C. The PES signal is generated by alternating between the P and Q components according to which one is more linear to provide a PENS signal with the greatest linearity.
As noted above, consumers are demanding greater disk storage capacities. which are being provided by greater track densities. One aid to achieving greater track densities is the magneto-resistive (M-R) head. The M-R head can generally be made more narrow than the conventional inductive head and therefore permits a "write wide/read narrow" track writing method that is conducive to achieving greater track densities. Unfortunately the relatively narrow M-R head has operating characteristics that produce a PES signal that has poor linearity which can lead to poor servo performance. A non-linear PES signal means that the amplitude of the PES signal does not vary linearly as the M-R head is moved from one edge of a track to the other edge of the track. This makes it more difficult to assess head location based on the PES signal. In addition the physical configuration of the M-R head can include a static offset error between the read and write heads.
The poor linearity of the M-R head signal has been somewhat overcome by recording the servo pattern at a reduced track pitch, typically performing servowriting at a 1/3 or 1/4 track pitch. That is. a single track of the servo pattern is written in increments of 1/3 or 1/4 pitch. This has the effect of increasing the read head width relative to the servo track and increases the linearity of the PES signal. Servowriting at a reduced track pitch also increases the time spent performing the servowriting, which increases manufacturing costs. In addition, alternating between the primary P and quadrature Q components of the readback signal for linearity must occur more frequently. This process is called stitching and can generate signal noise that leads to inaccuracies. The PES stitching also can increase track misregistration (TMR) error which is the error that occurs when the head position otherwise indicated by the PES is offset from the true position of the head relative to the track.
Attempts have been made to provide a more linear PES signal. For example, it has been found that a more linear PES can be provided from high track density disks if an alternative to the P and Q definitions above are used. In particular, rather than use P=B-A and Q=D-C to form the PES signal, a more linear signal can be provided for high track densities by demodulating the servo signal using a difference of sums calculation involving A, B, C, and D, such as primary and quadrature definitions specified by P=(A+B)-(C+D) and Q=(C+D)-(A+B). See, for example, U.S. Pat. No. 5,381,281 to Shrinikle et at.
In the interest of controlling PES signal phase alignment, which can become a problem as the linear density of the PES pattern increases, various seamless servo patterns have been proposed. Such seamless servo patterns are written in one pass at a time without erasure. using a single M-R read/write head, and improve the linearity of the PES signal. More particularly, as the track density increases, the servo pattern frequency must increase to keep the frequency of the servo burst in the highest sensitivity frequency band of the servo processing channel. The increased servo pattern frequency makes phase alignment of the servo pattern bursts during servowriting more difficult. A seamless pattern is easier to record because no phase alignment between adjacent pattern component burst stripes is necessary. Thus, seamless patterns are often used with high track density disks.
Unfortunately because the width of a seamless servo pattern is determined solely by the width of the servowriting head, the width of a seamless servo pattern has a variation equal to the servowrite head width variation. For heads to be used in very high track density applications the tolerances in the read and write head widths are sufficiently variable that it is difficult to select an appropriate PES signal demodulation equation that is optimal for all heads.
Thus, the quadrature P and Q signal components provide a satisfactory PES signal for many applications. In the case of high frequency servo patterns used with high track densities, a difference of sums demodulation scheme has been found to be useful but M-R head manufacturing tolerances make it difficult to know which is better for a given head.
From the discussion above, it should be apparent that there is a need for a disk drive with servo demodulation that can account for differences in track densities and head widths to provide optimal servo demodulation for the track density and head width of the particular disk drive. The present invention satisfies this need.